Combustion appliances burn fuels for heating, cooking, or decorative purposes. Examples of combustion appliances include boilers, furnaces and water heaters. Combustion, or burning, is a high-temperature exothermic chemical reaction between a fuel and an oxidant, which in the case of combustion appliances is atmospheric oxidant. Common fuels used by these appliances are natural or liquefied petroleum gas and fuel oil.
The gaseous product of combustion is comprised primarily of water vapor, carbon dioxide and heat, and also harmful compounds such as carbon monoxide and nitrogen dioxide. It is desirable to vent this gaseous product to the outside atmosphere of a building or house (hereafter “building”), which is why combustion appliances all have flues (ducts or pipes) which dispose the combustion product to the outside atmosphere.
High efficiency combustion appliances, known as “condensing” appliances, extract additional heat from the gaseous combustion product before it is released to the outside atmosphere, by condensing the water vapour in the gas to liquid water, thus recovering its latent heat of vaporization. An exhaust gas of lower temperature is then vented to the outside. Condensing appliances range in efficiency from 83% to 98%, thus a typical increase of efficiency in a condensing appliance over a non-condensing appliance can be as much as 10-15%.
Traditional (i.e., non-condensing) combustion appliances were vented through the roof, as the buoyancy of the hot combustion gas could be relied upon to move the gas up through a chimney flue and out of the building. However, because so much heat is removed from the combustion gas of condensing appliances, the gas has a lower-temperature and these appliances must use forced venting systems to ensure proper exhaust gas flow. Because of this forced venting and water condensation, high-efficiency appliances cannot use a natural draft chimney. All condensing appliances must be individually vented in North America, and can be vented through the sidewall of a building, or through the roof. In Europe group venting of appliances is practiced.
An “exhaust” vent is a conduit which vents exhaust gas from a condensing appliance to the outside atmosphere. Sidewall exhausts are commonly discharged close to the ground, adjacent to the building and on the same side of the building. There are two basic systems for installing a two-pipe configuration (as opposed to a one-pipe configuration where air for combustion is taken from the room that the appliance is in). One is a side-by-side configuration, which uses adjacent conduits for the exhaust and air intake that terminate at two separate locations (see e.g., FIG. 1A which shows sidewall venting). The second is a concentric pipe configuration that has an exhaust pipe inside air intake pipe, both of which terminate at the same location (see e.g., FIG. 1B which shows sidewall venting). Concentric pipe configurations commonly comprise an anti-mixing baffle to isolate the air intake opening from the exhaust vent opening. These two basic venting configurations can be used to vent condensing appliances vertically, for example through an existing chimney chase, or horizontally, for example through a sidewall.
On cold days in cooler climates, the moisture plume from the exhaust vent of condensing appliances is clearly visible. If sidewall venting is used, warm moist exhaust gas moves up the wall of the building to any overhangs above, and/or is ejected into the space between adjacent buildings. As it encounters the cooler surfaces of a building it can condense and cause ice buildup. In areas where there is little distance between adjacent buildings, the exhaust gas from one building can cause ice buildup on the adjacent building. It is a common practice to install a redirection fitting on the vent termination to minimize the amount of moisture impacting the buildings opposite the vent termination. This practice increases the ice condensing surface area of the vent termination and additional icing takes place as a result of this practice. If two or more buildings vent exhaust gas into the same space, the air in that space can become very moist, exacerbating the buildup of ice. Ice buildup can also occur in and around the exhaust vent itself, impeding venting of the exhaust gas—not only will the appliance no longer function, it becomes a safety concern.
US 2002/0123305 by Tocher describes a fresh air intake that is allegedly designed to avoid plugging up during the winter with frost and snow. The intake comprises galvanized metal with a metal screen that has ¼″ open squares.
U.S. Pat. No. 6,102,030 to Brown et al. describes a concentric furnace exhaust and intake configuration allegedly designed to avoid mixing of exhaust gas with intake air. In this device, the exhaust vent is configured as a nozzle to accelerate the exhaust gas away from the intake pipe and it is also disposed asymmetrically to further discourage the mixing of combustion air and exhaust gas.
US 2009/0017746 by Clemenz et al. describes an apparatus used for preventing the accumulation of snow, ice, frost and hail into or out of a building. The apparatus has a screen that is electrically connected to a heating device. The apparatus may be incorporated into new pipes or attached onto existing pipes.
U.S. Pat. No. 8,327,836 to Brown et al. describes a combined air intake and exhaust vent assembly that is attached to a pair of pipes extending outward from the side of a building. In this assembly the vent and intake terminals are separated and placed in side-by-side relationship.
FIG. 2 shows a drain-waste-vent system, known in the prior art, which removes sewage and greywater from a building, and regulates air pressure in the waste-system pipes to aid free flow. All of the water drains in a house are generally connected to a main vent stack which goes up through the attic and sticks out on the roof of the house. This vent pipe, or vent stack, allows air to flow in the drain system and prevents water flowing down a drain in the house from trying to draw air through drain traps under sinks and in toilets. Without vent piping, or when a vent pipe is blocked, the water in the drain traps can be sucked out, letting sewer gas into the house. This gas, primarily composed of air, water vapor and organic gases, will have an obvious “sewer” smell; however, some harmful gases are odorless and the vent gases could be compromised of gases containing H2S from a septic system or a flammable vapor such as gasoline, if it were allowed to enter the sanitary sewer system serving the household which the vent stack is connected to.
Blockage of the vent stack can occur when ice builds up at the top of the stack, called “ice capping”. There are three mechanisms that can cause icing of the vent stack, and they can occasionally work together:                wet blowing snow with at just the right temperature conditions, can deposit the ice on the top;        hot water miming down a drain, for example from a hot shower, gives off steam. This steam finds its way up the plumbing vent and out the top of the house. During extended periods of very cold weather, particularly in Northern climates, the upper portion of the vent pipe is much colder than the portions of the pipe that are in the house. The water in the steam condenses and then freezes to the top of the vent pipe before it escapes out the top; and        hard, drifted snow can plug the end of the vent stack pipes.        
A traditional way to remove the ice cap is to climb up on the roof and pour boiling water down the pipe. Occasional ice capping may be avoided by wrapping insulation around the pipe in the attic space right up to the underside of the roof to keep the steam hotter longer. Or, an insulated box can be built around the vent stack on the roof. In situations where insulation isn't sufficient, a thermostatically controlled electric heating cable (heat tape) which keeps the end of the vent pipe warm can be used, optionally with insulation around the cable. However, if not installed correctly this can cause fires. Another possible solution is a total replacement for the top of the plumbing stack using an apparatus that starts inside the attic and is connected to electricity inside the attic to avoid running wires to or around the vent.
There remains a need for a means of reducing or eliminating the deposition of frost and ice at the terminus of an exhaust vent of condensing appliances, at the terminus of a plumbing stack vent, or at the terminus of other vents, particularly in colder climates. It would be desirable if the solution to these problems did not require the use of an external power source, but rather could be self-sustaining.